publish: github release plugin

I've PR'd this against upstream drone, but it's stagnating. I'd like to start testing this in our drone setup.

Can we list here what a 422 error means? I'm assuming from context that it means that there's already a release for that tag. Also "Ignore 422 errors" makes it sound like it should proceed with the release in spite of 422 errors, but I don't think that's the behavior we want (or is implemented below)